In the manufcture of plastics blanks, it is known to inject plastics material into a cavity formed between a separable outer mould and a fixed mandrel. Thus the injection of the plastics material takes place centrally of the blank and at the end of the cavity adjacent the free end of the mandrel.
It has become apparent that, with high pressure injection of the plastics material, the mandrel can be bent or warped because of uneven rates of flow of the plastics material around the mandrel. Blanks with considerably varying wall thickness and strength are produced by bending of the mandrel and this causes difficulties in the subsequent blow-moulding process. The warp on the mandrel is all the more apparent, the longer the mandrel. The length of the mandrel can be five to ten times as much as its diameter. With continued use of blanks with varying wall strengths and thicknesses the quality of the bottles being manufactured is considerably reduced and the productivity of the bottle producing machine suffers accordingly.
Moreover, streaks and swirls can form in the blanks as a result of uncontrolled injection of the plastics material, which streaks and swirls adversely affect the quality of the blanks.